This invention relates generally to carburetors and more particularly to a modular diaphragm type carburetor.
Typically, carburetors have been used to supply a fuel and air mixture to both four stroke and two stroke internal combustion engines. For many applications where small two stroke engines are utilized, such as hand held power chain saws, weed trimmers, leaf blowers, garden equipment and the like, carburetors with both a diaphragm fuel delivery pump and diaphragm fuel metering system have been utilized. Typically, these carburetors comprise a main body having a pair of end caps each of which traps a separate one of the fuel pump diaphragm and fuel metering diaphragm against the carburetor body and defines various fuel pump chambers or fuel metering chambers.
To transfer fuel from the fuel pump assembly to the fuel metering system and thereafter to a throttle or venturi bore in the carburetor body for delivery of a rich fuel and air mixture to the engine, as well as to provide air flow and pressure control signals through the carburetor, a plurality of passages must be formed in the carburetor body and a number of pockets or recesses are formed in the various chambers within the body to facilitate communicating desired passages with each other. This machining is intricate, time consuming and therefore greatly increases the cost to manufacture the carburetors. Further, cavities or recesses must also be provided to receive valves or other components between the fuel metering diaphragm and the body of the carburetor. These cavities or recesses can trap vapor bubbles which coalesce to form large vapor bubbles. The large vapor bubbles are eventually drawn through the carburetor and delivered to the engine making the fuel and air mixture delivered to the engine temporarily overly lean and contributing to poor engine performance. Still further, the various components of the carburetor are assembled in many directions which increases the manual labor needed to assemble the carburetor and thereby increases the cost to manufacture and assemble them.
In a conventional carburetor having a main body wherein a plurality of passages and openings are machined, it is extremely difficult and often not possible to use a particular carburetor body on more than one engine family. Still further, due to the difficulty in machining and assembling the carburetor body, there is a significant variation from carburetor to carburetor. This carburetor to carburetor variation must be compensated for by initially calibrating each carburetor to its desired performance which can be difficult to do with the conventional needle valve assembly and fuel metering arrangement in conventional carburetors.
A modular diaphragm carburetor is provided which has a plurality of plates each with generally planar faces adapted to be mated and releasably connected together to facilitate manufacturing and assembling the carburetor and to permit various plates and components of the carburetor to be used in other carburetors designed for use with different engine families. By providing a plurality of mated together plates, the machining of the passages through the carburetor is made dramatically easier compared to the machining of a carburetor having a single body with end caps. Still further, the modular diaphragm design permits different plates and/or components of the carburetor to be used with other components to provide a carburetor having different performance characteristics and suitable for use with a different engine family. Therefore, a wide range of carburetors can be provided which have many of the same components to reduce the overall part count and to more economically manufacture and assemble a wide range of carburetors.
To also increase the flexibility of the carburetor, an improved system is provided for controlling the operating vacuum pressure of a fuel metering system of the carburetor. By changing the operating vacuum of the fuel metering system, the flow characteristics through the carburetor can be changed as desired to suit particular engine families. Desirably, a valve which controls the flow of fuel to a fuel metering chamber in the carburetor can be opened by a disk responsive to movement of the fuel metering diaphragm to control the flow of fuel into the fuel metering chamber. Further, the working length of a spring yieldably biasing the valve to its closed position can be changed to change the force acting on the inlet valve. With this arrangement, the diameter, construction and mass of the disk, the flexibility of the fuel metering diaphragm, the design of the inlet valve and its seat, and the magnitude of the spring force biasing the inlet valve to its closed position all contribute to the average magnitude of the vacuum at which the fuel metering chamber operates. Therefore, the average operating vacuum of the fuel metering chamber can be varied by varying any one or more of the above components to ensure proper operation of the carburetor on various engine families.
It is also important that the operating vacuum of the fuel metering To chamber be consistent from carburetor to carburetor on the same engine family. With all other factors being essentially equal, the operating vacuum of the metering chamber can be readily altered by modifying the working length of the spring biasing the inlet valve to change the force exerted on the inlet valve by the spring. In conventional carburetors, to change the spring force acting on the inlet valve, it was necessary to replace the spring with another spring having a different spring rate. Therefore, permitting the adjustment of the working length of the spring facilitates calibrating the carburetor for consistent performance on the same engine family and also facilitates use of the carburetor on various engine families.
By changing the operating vacuum of the fuel metering chamber, the fuel flow characteristics of the carburetor are changed. Desirably, the fuel flow characteristics can be controlled in this manner without the use of any needle valves typically found in conventional carburetors, to facilitate calibrating the carburetor and ensure that it is tamper proof so that an end user cannot easily adjust the carburetor out of a desired operating range. If desired, needle valves may still be employed to control in part the fuel flow characteristics of the carburetor if desired for a particular application.
Objects, features and advantages of this invention include providing a carburetor which has a body formed from a plurality of plates to facilitate manufacture and machining of the various passages in the carburetor, facilitates adjustment from carburetor to carburetor for use with the same engine family, facilitates adjustment of the carburetor for use on different engine families, enables use of various carburetor components in assembly of a different carburetor for a different engine family, facilitates adjustment of the operating pressure of a fuel metering chamber, permits final assembly from a single direction, permits various subsystems of the carburetor to be tested independently of one another before final assembly, permits a fuel pump portion of the carburetor to be formed without machining, permits an increased fuel filter area without degradation of performance of the carburetor, permits use of flat, non-convoluted diaphragms, reduces cavities or pockets in fuel chambers and fuel passages to reduce vapor bubble collection, permits direct access to a spring biasing a fuel metering inlet valve to permit its working length to be adjusted, is of relatively simple design and economical manufacture and assembly, reliable, durable and has a long useful life in service.